Sure, there can be physical memory at address zero, there can be virtual memory at address zero. (Most modern operating systems and programming tools place a deathtrap for misbehaving code there instead.) There can be no physical memory at address 0 at all, though it is uncommon, and other gaps in addresses. RAM can technically start elsewhere, and lower part of memory can be taken by ROM or other hardware. (For example, see how authors of breadboard computers reason about the choice of system memory layouts and address line operation.) NULL value itself can be different from zero. Therefore, C standard defines nothing about null pointer dereferencing.
On the other hand, most people expect the usual layout that grows from zero upwards. CPU designers are people, too, and make similar assumptions about layout and operation of memory (if they have to), which then affect systems that use their processors. Also, with all its compatibility, C still presumed certain class of hardware. It was not invented as a language for every microprocessor and microcontroller and each odd memory model, the microcontroller evolution was instead affected by alignment with coding in C.
This ugly hack (using the same object to hold the address value that can be operated on, and its own validity information) might be the most well known.